1. Field of the Invention
The present invention relates to a reflective mask used in extreme ultra violet (EUV) lithography.
2. Description of the Related Art
An inspection process wherein the accurate formation of a circuit pattern is inspected is usually conducted in a method for manufacturing a reflective mask for EUV lithography. In this inspection process, the accurate formation of a circuit pattern is inspected by irradiating a reflective mask with inspection light and detecting reflected light from the reflective mask. In the inspection process, defects may be easily detected when the ratio (contrast) of the reflectance of an absorber to the reflectance of a layer below the absorber (for example, a multilayer, a capping layer, or a buffer layer) is high.
In the inspection process, positional alignment between a reflective mask and an inspection apparatus is usually conducted for inspecting a circuit pattern. In the reflective mask, therefore, an alignment mark is usually formed in addition to a circuit pattern.
In EUV lithography, a conductive layer is usually formed on one side of a substrate opposite to the side of the substrate on which a circuit pattern and an alignment mark are formed, in order to hold the reflective mask on an electrostatic chuck of an exposure apparatus. In this conductive layer, a light-shielding material such as chromium is generally used, and the conductive layer is formed on the whole surface of the substrate. Accordingly, the surface of the reflective mask on which a circuit pattern and an alignment mark are formed should be irradiated with alignment light at the time of positional alignment, to detect reflected light from the alignment mark of the reflective mask.
However, in general, the alignment mark is relatively small so that the contrast maybe lowered, or depending on the wavelength of alignment light, the contrast may be significantly lowered, and thus there is a problem of difficulty in positional alignment.
Although not directed to the positional alignment between a reflective mask and an inspection apparatus in inspecting a circuit pattern of the reflective mask, a following method of positional alignment between a reflective mask and an exposure apparatus in EUV lithography has been proposed: a reflective mask having an alignment mark and a conductive layer formed on one side of a substrate opposite to the side on which a circuit pattern is formed is irradiated with alignment light, and reflected light from the alignment mark is detected (see, for example, Japanese Patent Application Laid-Open (JP-A) 2005-11914). However, the method described in JP-A 2005-11914 is a method of detecting an alignment mark formed on one side of a substrate, and even if this method is applied to positional alignment between a reflective mask and an inspection apparatus in inspecting a circuit pattern of the reflective mask, the positional relationship between the surface of the substrate and the alignment mark on the other side of the substrate may not be said to be accurate, thus making the alignment difficult.
Although directed neither to EUV lithography nor to the positional alignment between a reflective mask and an inspection apparatus in inspecting a circuit pattern of the reflective mask, a following method of alignment between a reflective mask and an exposure apparatus in X-ray projection exposure has been proposed: a reflective mask in which an alignment mark consisting of a light-shielding portion and a light-transmitting portion is formed on a region other than where an absorber pattern is formed such as in the periphery of the reflective mask is used to detect the alignment mark by transmission (see, for example, JP-A 2000-77306). However, the method described in JP-A 2000-77306 is not directed to EUV lithography, and when a conductive layer is formed on one side of the substrate opposite to the side on which a circuit pattern is formed, the alignment mark may not be detected by transmission.